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Experimental studies on the competitive balance between two central European roadside grasses with different growth forms .1. Field experiments on the effects of mowing and maximum leaf temperatures on competitive ability

The weakly-competitive grass, Puccinellio distons (Jacq.) Parl., and the highly competitive grass, Elymus repens (L.), coexist on roadsides in Central Europe which are regularly mowed. The effect of mowing on this existence was evaluated in situ at the roadside and in experimental field plots under non-limiting water and nutrient conditions. In four experimental garden plots, tussocks of P. distons were transplanted into monocultures of E. repens early in the growing season. After establishment, two of the plots were mowed and two left unmowed. In the unmowed plots, P. distans was quickly overtopped and died, while in mowed plots, P. distons was able to persist throughout the growing season. Despite persistence in the mowed plots, the number of tillers of P. distons declined throughout the growing season as numbers of tillers of E. repens increased. It was concluded that mowing could enhance coexistence in situations of unlimited nutrients and water where the faster growing and aggressive E. repens would exclude P. distans without mowing. At the roadside, six plots were established at a site containing both species early in the growing season. Weekly mowing was performed on three of the plots while the others were left unmowed. In mid-july, when the grasses were beginning to senesce and had produced seedheads, foliage area for P. distans was significantly higher in the mowed than the unmowed plots while the reverse was true for E. repens. While P. distons had higher foliage area in the mowed plots, it was able to persist to seedhead production in the unmowed plots. Simulations conducted with a multispecies canopy photosynthesis model indicated that reductions in carbon gain for P. distons due to shading by foliage of E. repens did not correspond well with foliage area for P. distans at the start of experiment or in the mowed and unmowed plots in mid-july. In both the mowed and unmowed plots, the portion of foliage consisting of P. distans increased with closeness to the roadway and corresponded inversely with soil depth, At soil depths of greater than 15 cm, P. distons did not occur. More effective exploitation of shallow soil may shift the competitive balance toward P. distons and be a significant factor in the coexistence of these two species in the shallow soils at the roadside. Differences in leaf temperatures at the roadside that might result from different leaf widths were also evaluated. The narrower-leaved P. distans was hypothesized to have lower midday leaf temperatures at sites close to the asphalt and perhaps be closer to the temperature optimum for photosynthesis during warm summer days. In situ leaf temperature measurements made with small thermocouples attached to intact leaves, however, were not significantly different for the two species, and coexistence was not likely to be affected by leaf temperatures.